Guillem Femenias

BER performance of linear STBC from orthogonal designs over MIMO correlated Nakagami-m fading channels

This paper presents the evaluation of the average bit error rate (BER) performance of linear space-time block codes (STBC) from orthogonal designs over correlated identically distributed Nakagami-m fading channels. Starting from the moment-generating function (MGF) of the multipath component signals at the antenna array elements, analytical expressions of the BER performance for both integral and nonintegral Nakagami-m fading parameters are derived. Closed-form expressions of the spatial cross-correlation function for mobile nonfrequency selective Nakagami-m fading multiple-input-multiple-output (MIMO) channels are obtained, which are valid for small angle-of-arrival (AOA) spread. In this expressions, various parameters of interest, such as the mean AOA of the signal, AOA spread, and array configurations, are all taken into account. The effects of antenna array configuration and the operating environment (mean AOA, AOA spread, Nakagami fading parameter) on the BER performance of the system are illustrated by several numerical examples.

Cross-Layer Design of Adaptive Multirate Wireless Networks Using Truncated HARQ

To provide heterogeneous quality-of-service (QoS) guarantees to applications, most wireless communications standards combine the error-correcting capability of hybrid automatic repeat request (HARQ) protocols at the data link control layer with the adaptation ability of adaptive modulation and coding (AMC) strategies at the physical layer. In this paper, a novel cross-layer multidimensional discrete-time Markov chain (DTMC)-based queuing model is developed to jointly exploit the capabilities of HARQ and AMC. The analytical DTMC-based model, which generalizes and extends previous results on this topic, stems from a comprehensive consideration of packet multimedia traffic sources modeled as discrete-batch Markovian arrival processes, finite queue-length systems, truncated HARQ protocols, AMC strategies, and a wireless channel first-order 2-D Markov model that relies on the amplitude and rate of change of the fading envelope. Based on the stationary state probability distribution of this multidimensional DTMC, closed-form analytical expressions for performance metrics such as throughput, average packet delay, and packet loss rate, which were caused either by buffer overflow or by exceeding the maximum number of allowed retransmissions, are derived. Furthermore, the proposed analytical framework is used to formulate multidimensional and simplified 2-D constrained optimization problems aiming at maximizing the system throughput under prescribed QoS constraints. Computer simulation results are carried out to verify the validity of the proposed analytical model and to quantify the performance gain due to cross-layer optimization and the use of truncated HARQ protocols.

Using Two-Dimensional Markov Models and the Effective-Capacity Approach for Cross-Layer Design in AMC/ARQ-Based Wireless Networks

This paper proposes a novel framework for the cross-layer analysis and design of wireless networks combining adaptive modulation and coding (AMC) at the physical layer with an automatic repeat request (ARQ) protocol at the data-link layer. Most previous works rely on first-order amplitude-based finite-state Markov chains (AFSMCs) to model the physical layer. It is shown that these models present several deficiencies that could compromise the design of higher layer protocols. Thus, a physical-layer first-order 2-D Markov model using both the amplitude and the rate of change of the fading envelope is presented. Based on this multidimensional physical-layer Markov model, the quality-of-service (QoS) performance at the data-link layer is investigated through the use of two different approaches. The first one relies on an analytical framework based on a discrete-time Markov chain (DTMC) that jointly describes the statistical behavior of the arrival process, the queueing system, and the physical layer. The second one is based on the effective-bandwidth and effective-capacity theories. Both the DTMC-based and the effective-bandwidth/capacity-based approaches are analyzed and compared in combination with our proposed physical-layer first-order 2-D Markov model in a cross-layer design aiming to satisfy the required average packet loss probability constraint by maximizing the average throughput of the system. Numerical results show that our proposed framework represents a significant improvement over previous models.

Design and implementation of a wide-band real-time mobile channel emulator

A new wide-band mobile channel emulator for the CODIT project is designed and implemented. The UMTS code-division testbed (CODIT R2020) is a research project within the European RACE-II program set up by the Commission of the European Community. Our goal is to be able to simulate in the laboratory, in real time, the multipath propagation found in the mobile radio channel. As code-division multiple access (CDMA) is the access technique within the CODIT project, it was realized that the channel emulator must have simultaneously good delay resolution between propagation paths and long duration of the impulse response. These considerations led to a very flexible channel emulator specifically designed to host the new wide-band channel models developed within the CODIT project. Our emulator features three independent inputs and two outputs, up to 20 complex propagation paths, 10-MHz radio frequency (RF) bandwidth, a delay resolution of 50 ns, and a maximum duration of the channel impulse response of 80 /spl mu/s. Starting with an explanation of the global structure of the new channel emulator, we derive the optimum design of the interpolation procedures and present the main implementation issues arising from our initial architecture. Finally, we report the results of the laboratory tests of the first prototype of the channel emulator.

On the design of uplink and downlink group-orthogonal multicarrier wireless systems

Group-orthogonal multicarrier code-division multiple access (GO-MC-CDMA) has been proposed as an attractive multiplexing technique for the uplink segment of wireless systems. More recently, a variant of this scheme has also been proposed for the downlink. This paper presents a unified bit error rate (BER) performance analysis of group-orthogonal wireless systems when using maximum likelihood (ML) multiuser/multisymbol detection covering both link directions. Valuable design rules regarding the number of subcarriers per group and the selection of spreading codes are derived. Simulations results using realistic system parameters and ETSI BRAN channel models are also presented which serve to validate the analytical results.

Cross-Layer Fast Link Adaptation for MIMO-OFDM Based WLANs

This paper considers the use of cross-layer fast link adaptation (FLA) for WLANs employing a MIMO-OFDM physical layer. A packet error rate (PER)-based FLA technique that, without loss of generality, makes use of the exponential effective SNR mapping (EESM) is proposed. Additionally, an FLA scheme relying on bit error rate (BER) metrics is introduced that simplifies the link adaptation procedure without any significant performance degradation. Results show that both PER- and BER-based FLA techniques optimize the data throughput while satisfying prescribed quality of service constraints. Channel estimation errors have also been considered, revealing the importance of good channel estimators in order for FLA strategies to work satisfactorily.

Analysis of predetection diversity TCM-MPSK and postdetection diversity TCM-MDPSK systems on a Rayleigh fading channel

In this paper, the bit error rate (BER) performance of predetection diversity trellis coded multilevel phase shift keying (TCM-MPSK) and postdetection diversity trellis coded multilevel differential phase shift keying (TCM-MDPSK), transmitted over a Rayleigh fading channel is presented. Novel analytical upper bounds using the transfer function bounding technique are obtained and illustrated by several numerical examples. A new asymptotically tight upper bound for the maximal ratio combining (MRC) diversity schemes is also derived. In order to analyze practical TCM schemes (four or more states), only trellis codes holding uniform error property (UEP) and uniform distance property (UDP) are considered, enabling the encoder transfer function to be obtained from a modified state transition diagram, having no more states than the encoder itself. Monte-Carlo simulation results, which are more indicative of the exact system performance, are also given. >

SR ARQ for adaptive modulation systems combined with selection transmit diversity

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. In this paper, both throughput and packet error rate are analyzed for a selective-repeat ARQ scheme based on a constant-power variable-rate adaptive M-QAM system combined with selection transmit diversity over multiple-input multiple-output Markovian-Nakagami channels. In this analysis, the impact of using outdated and/or imperfect channel state information on the performance of the system is considered.

Unified approach to cross-layer scheduling and resource allocation in OFDMA wireless networks

Orthogonal frequency division multiple access (OFDMA) has been selected as the core physical layer access scheme for state-of-the-art and next-generation wireless communications standards. In these systems, scheduling and resource allocation algorithms, jointly assigning transmission data rates, bandwidth and power, become crucial to optimize the resource utilization while providing support to multimedia applications with heterogeneous quality of service (QoS) requirements. In this article, a unified framework for channeland queue-aware QoS-guaranteed cross-layer scheduling and resource allocation algorithms for heterogeneous multiservice OFDMA wireless networks is presented. The framework encompasses different types of traffic, uniform and continuous power allocation, discrete and continuous rate allocation, and protocols with different amounts of channel- and queue-awareness. System parameters and QoS requirements are projected into utility functions and the optimization problem is then formulated as a constrained utility maximization problem. Optimal solutions for this problem are obtained for the uniform power allocation schemes, and novel quasioptimal algorithms are proposed for the adaptive power allocation strategies. Remarkably, these techniques exhibit complexities that are linear in the number of resource units and users. Simulation results demonstrate the validity and merits of the proposed cross-layer unified approach.

Reverse Link Performance of a DS-CDMA System With Both Fast and Slow Power Controlled Users

In this paper the performance of the reverse link of a multicell DS-CDMA system with coexisting open-loop and closed-loop power controlled users transmitting heterogeneous traffic is analyzed. Real-time and non-real-time traffic performance expressions are obtained. The analysis includes a different channel coding scheme for each traffic type selected in accordance to their specific QoS requirements. Moreover, the study takes into account the effect of frequency-selective Nakagami-m fading with arbitrary parameters, correlated log-normal shadowing, power control imperfections and selection-based macroscopic diversity. How the power control imperfections, the number of resolvable paths, or the proportion of simultaneous open-loop and closed-loop power controlled users affect the reverse link capacity of the system is discussed in detail. Analytical results are also given for systems with different processing gains and for propagation environments with different multipath intensity profile (MIP) distributions.